Thursday, 7 March 2013

Believing a treatment will work, even if in reality it is entirely inert, can lead to profound beneficial changes. This is the wonder of the placebo effect and most of us have heard it discussed in relation to helping people with physical ailments.

Less explored is the potential the effect could have in other contexts. There are some examples, such as a paper published two years ago by Sophie Parker showing that memory performance was enhanced when participants thought they'd taken a cognition-enhancing drug, even though they hadn't. However, this still has parallels with a medical placebo because of the use of a sham drug.

Now Ulrich Weger and Stephen Loughnan have gone a step further, by showing that a drug-free placebo intervention boosted the general knowledge performance of a group of students. Pub quizzes might never be the same again!

The researchers started by showing 20 participants the answer on-screen to several multiple-choice test items, just prior to the arrival of each of the questions (examples included Pi, and the painter of La Guernica). Next, the researchers gradually reduced the time the answers were presented, until they became completely invisible. This was to demonstrate the principle of subliminal presentation and laid the ground for the experiment proper.

For the real test, involving a new set of 20 questions, Weger and Loughnan told the same participants that for each test item, the correct answer would be presented to them subliminally beforehand, just as in the earlier demonstration. "We further advised them that although they could no longer consciously recognise what was written, their unconscious would still be able to pick up the correct answer," the researchers explained, adding that they told the participants to go with their intuition because "on some level you already know the answer". In reality, however, no answers were presented subliminally, just random letter strings. The researchers call this a "bogus priming method". Participants gave their answers via a paper sheet.

The key finding is that the test performance of the placebo participants significantly outstripped the performance of a control group of twenty students who undertook the test prelims, but were not told the answers would be shown to them subliminally during the test proper (average 9.85 correct out of 20 vs. 8.37 correct; a large effect size of d=0.813). The average age of the 40 participants was 20 years and there were 32 women. The priming placebo effect held when controlling for participant age and gender.

What was going on? The placebo intervention "cannot have expanded the individual's knowledge or storage capacities," the researchers said. "What is more likely to have happened is a weakening of inhibitory mechanisms that normally impair performance on a task - for example, self-incapacitating anxieties that previously taxed cognitive resources." The placebo might also have "primed a success orientation," the researchers said, which may have affected the participants' behaviour accordingly, including increasing their persistence.

Weger and Loughnan are excited about the possibilities their placebo approach might have for testing people's performance in situations where their anxiety might otherwise interfere with achieving their true potential. The real life benefits of this new test-performance placebo will likely depend on the duration of its effect, something the researchers plan to test in future research. Another issue for applying these findings in real life is whether the effect still occurs when people know the trick (at least one previous study has documented a placebo effect that can work without deception). The researchers are optimistic - "we speculate that even the one-off realisation to have skills and resources that the participant was not previously aware of can be a significant insight that may alter the individual's self-perception and self-talk."

Skeptics will no doubt be concerned about the small size of the sample in the current study and the narrow demonstration of the effect. There's clearly a need for replications! As Weber and Loughnan acknowledged, we also need to know more about the mechanisms underlying the improved test performance.

Regarding "coincidence", the results were statistically significant so the probability of the effect being a fluke is about 1.6%, which is usually considered acceptable. (I realise that significance testing has limitations, but still the results should not be dismissed out of hand.) Although the difference between the groups was only about 1.5 points out of 20, this amounted to over 0.8 standard deviations, a substantial effect size. Considering that placebo priming cannot instil knowledge people don't already have, but only at best improve a person's ability to retrieve existing knowledge I don't think it would be reasonable to expect a huge absolute difference if the effect is genuine. One possible source of sampling bias that the authors did not address was if the groups differed in mean age or in gender balance. Standard research practice would be to ensure that each group had the same gender balance, but the authors do not specify if this was actually done or not. Other research studies have found that general knowledge tends to increase with age and that men tend to score higher on such tests than women, so inequalities in these variables could have effected the results. Since they were randomly assigned, the mean ages were most likely equal but it would have been good if this was reported. I agree that a larger sample would be desirable to ensure the effect is robust.

Interesting. I look forward to seeing this replicated on a larger scale.

A couple questions (sorry if the answers are obvious, but the article is paywalled):

- Were the full data distributions published? The summary only reported average scores, which I assume were the primary endpoint, but those are more sensitive to outliers than medians, especially with such a small sample. Since we’re only talking about 40 scores, it’s not unreasonable to expect they’d all be shown.

- (Related to the above: how does one get an average of 8.37 out of 20 scores?)

- Seconding the comment above, was there no review of the demographics of the 2 groups? It doesn’t seem

- Any discussion of negative priming towards the control group? Given that the test administrator was possibly unblinded (there must have been 2 scripts, one mentioning the intervention and one not), were any precautions taken to avoid biasing the controls? Negative priming’s effect at _reducing_ test scores seems somewhat well established, so this would seem to be a concern.

Relevant to your questions, I added this info to the post: "The average age of the 40 participants was 20 years and there were 32 women. The priming placebo effect held when controlling for participant age and gender."

There was no discussion of negative priming and few details given about the treatment of the control group, other than they undertook all the same prelims but were not fed the line about the answers being shown to the subliminally during the test proper. They were just told they'd be a flash before each new question to indicate the beginning of a new trial. There was no mention of the researchers being blinded to the conditions.

The full data distributions were not published. One control participant was omitted for failing to follow task instructions.